Apparatus and method for adaptive beam-forming in wireless communication system

ABSTRACT

Apparatuses and methods for maintaining an optimal beam direction in a wireless communication system are provided. The method for operating a receiving node in a wireless communication system includes, determining a first transmission beam is determined as a preferred transmission beam using a plurality of reference signals transmitted by a transmitting node, generating preferred transmission beam information, transmitting the preferred transmission beam information to the transmitting node, receiving transmissions from the transmitting node via the first transmission beam, and determining whether a change of a transmission beam is necessary. When the change of the transmission beam is determined to be necessary, generating a beam change request and transmitting the beam change request to the transmitting node.

PRIORITY

This application claims the benefit under 35 U.S.C. §119(a) of a Koreanpatent application filed in the Korean Intellectual Property Office onAug. 12, 2011 and assigned Serial No. 10-2011-0080676, the entiredisclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wireless communication system. Moreparticularly, the present invention relates to an apparatus and a methodfor adaptively performing beam forming in a wireless communicationsystem that uses a Super High Frequency (SHF) band.

2. Description of the Related Art

To meet continually increasing Radio Frequency (RF) data trafficdemands, wireless communication systems are evolving so as to supporthigher data transmission rates. For example, an emphasis in thedevelopment of a 4th Generation (4G) wireless communication system is animprovement in spectral efficiency in order to increase a datatransmission rate. However, it has become difficult to meet thecontinually increasing RF data traffic demands with only improvements inthe spectral efficiency.

The above problem may alternatively be addressed by employing a widerfrequency band. A frequency band used by a mobile communication systemof the related art is typically 10 GHz or less. The reason for this isthat it is very difficult to secure a wider frequency band at 10 GHz orless. However, a wider frequency band may be more easily secured in asuper high frequency band (e.g., 30 GHz through 150 GHz). The super highfrequency band may also be referred to as the millimeter (mm) wave band.

However, the use of the super high frequency band has a shortcoming inthat as a frequency for wireless communication increases, a radio pathloss increases. Accordingly, as the frequency for wireless communicationincreases, radio reach distance is correspondingly shortened, therebyresulting in a reduced service coverage area. However, there is atechnique to address this shortcoming, namely beam-forming. Beam-formingaddresses the radio path loss and increases the radio reach distance.

Beam-forming may be classified into a transmission beam-forming and areception beam-forming. The transmission beam-forming concentrates areach region of a radio wave in a specific direction. The reach regionof a radio wave may be concentrated in a specific direction using aplurality of antennas. Generally, a structure including the plurality ofantennas is referred to as an antenna array, and an individual antennaincluded in the antenna array is referred to as an array element. In acase where transmission beam-forming is applied, a transmission distanceof a signal increases in an intended direction, and simultaneously, thetransmission distance of the signal in directions other than theintended direction are minimized. Therefore, interference to other userslocated in directions other than the intended direction is reduced. Inthe reception beam-forming, a reception side concentrates reception ofradio waves from a specific direction using a reception antenna array.Accordingly, strength of a signal received from an intended directionincreases, and strength of a signal received from directions other thanthe intended direction are excluded, so that an interference signal froma direction other than the indented direction is effectively blocked.

As described above, in order to secure a wider frequency band,utilization of a super high frequency band, that is, a millimeter (mm)wave band, is expected. In this case, beam-forming may be employed toovercome a radio path loss. Therefore, techniques are desired foreffectively performing beam-forming in a mobile communicationenvironment where a user moves and a radio environment changes.

SUMMARY OF THE INVENTION

Aspects of the present invention are to address at least theabove-mentioned problems and/or disadvantages and to provide at leastthe advantages described below. Accordingly, an aspect of the presentinvention is to provide an apparatus and a method for effectivelyperforming beam-forming in a wireless communication system.

Another aspect of the present invention is to provide an apparatus and amethod for overcoming a limitation of a periodic beam obtainingprocedure in a wireless communication system.

Still another aspect of the present invention is to provide an apparatusand a method for performing a beam changing procedure in response to abeam selection error in a wireless communication system.

Yet another aspect of the present invention is to provide an apparatusand a method for detecting an occurrence of a beam selection error usinga reference signal other than a reference signal for a beam obtainingprocedure in a wireless communication system.

Still another aspect of the present invention is to provide an apparatusand a method for reselecting a preferred beam using a reference signalother than a reference signal used for a beam obtaining procedure in awireless communication system.

In accordance with an aspect of the present invention, a method foroperating a receiving node in a wireless communication system isprovided. The method includes determining a first transmission beam as apreferred transmission beam using a plurality of reference signalstransmitted by a transmitting node, generating preferred transmissionbeam information, transmitting the preferred transmission beaminformation to the transmitting node, receiving transmissions from thetransmitting node via the first transmission beam, determining whether achange of a transmission beam is necessary, and when the change of thetransmission beam is determined to be necessary, generating a beamchange request, and transmitting the beam change request to thetransmitting node.

In accordance with another aspect of the present invention, a method foroperating a transmitting node in a wireless communication system isprovided. The method includes transmitting a plurality of referencesignals through corresponding transmission beams, receiving preferredtransmission beam information from a receiving node, which was generatedby the receiving node based on the reference signals, identifying afirst transmission beam as a transmission beam for transmitting a signalto the receiving node based on the preferred transmission beaminformation, transmitting the signal to the receiving node via the firsttransmission beam, receiving a beam change request from the receivingnode, identifying a second transmission beam as the transmission beamfor transmitting the signal to the receiving node, and one of adding andchanging the second transmission beam as the transmission beam used fortransmitting the signal to the receiving node.

In accordance with further another aspect of the present invention, areceiving node in a wireless communication system is provided. Thereceiving node includes a controller for controlling a transmitter and areceiver, for determining a first transmission beam as a preferredtransmission beam using a plurality of reference signals transmitted bya transmitting node, for generating preferred transmission beaminformation, for controlling to transmit the preferred transmission beaminformation to the transmitting node, for controlling to receivetransmissions from the transmitting node via the first transmissionbeam, for determining whether a change of a transmission beam isnecessary, and when the change of the transmission beam is determined tobe necessary, for generating a beam change request and for controllingto transmit the beam change request to the transmitting node; thetransmitter for transmitting the preferred transmission beam informationand the beam change request to the transmitting node; and the receiverfor receiving the transmissions from the transmitting node.

In accordance with further yet another aspect of the present invention,a transmitting node in a wireless communication system is provided. Thetransmitting node includes a beam-forming unit for applying one or moretransmission beams for the transmission of a plurality of referencesignals to a receiving node and the transmission of a signal to thereceiving node, a receiver for receiving preferred transmission beaminformation and a beam change request from the receiving node, and acontroller for controlling the beam-forming unit and the receiver, forcontrolling to transmit the plurality of reference signals throughcorresponding transmission beams, for controlling to receive thepreferred transmission beam information from the receiving node, whichwas generated by the receiving node based on the reference signals, foridentifying a first transmission beam as a transmission beam fortransmitting the signal to the receiving node based on the preferredtransmission beam information, for controlling to transmit the signal tothe receiving node via the first transmission beam, for controlling toreceive the beam change request from the receiving node, for identifyinga second transmission beam as the transmission beam for transmitting thesignal to the receiving node, and for one of adding and changing thesecond transmission beam as the transmission beam used for transmittingthe signal to the receiving node.

Other aspects, advantages, and salient features of the invention willbecome apparent to those skilled in the art from the following detaileddescription, which, taken in conjunction with the annexed drawings,discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of certainexemplary embodiments of the present invention will be more apparentfrom the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a view illustrating a beam region depending on a base stationand a terminal in a wireless communication system according to anexemplary embodiment of the present invention;

FIG. 2 is a view illustrating an example where a beam selection erroroccurs in a wireless communication system according to an exemplaryembodiment of the present invention;

FIG. 3 is a view illustrating an example of preferred beam determinationin a wireless communication system according to an exemplary embodimentof the present invention;

FIG. 4 is a view illustrating signaling for a beam obtain procedure anda beam change procedure in a wireless communication system according toan exemplary embodiment of the present invention;

FIG. 5 is a view illustrating signaling for a beam obtain procedure anda beam change procedure in a wireless communication system according toan exemplary embodiment of the present invention;

FIG. 6 is a flowchart illustrating a procedure for operating a terminalin a wireless communication system according to an exemplary embodimentof the present invention;

FIG. 7 is a flowchart illustrating a procedure for operating a basestation in a wireless communication system according to an exemplaryembodiment of the present invention;

FIG. 8 is a block diagram illustrating a terminal in a wirelesscommunication system according to an exemplary embodiment of the presentinvention; and

FIG. 9 is a block diagram illustrating a base station in a wirelesscommunication system according to an exemplary embodiment of the presentinvention.

Throughout the drawings, like reference numerals will be understood torefer to like parts, components and structures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following description with reference to the accompanying drawings isprovided to assist in a comprehensive understanding of exemplaryembodiments of the invention as defined by the claims and theirequivalents. It includes various specific details to assist in thatunderstanding but these are to be regarded as merely exemplary.Accordingly, those of ordinary skill in the art will recognize thatvarious changes and modifications of the embodiments described hereincan be made without departing from the scope and spirit of theinvention. Also, descriptions of well-known functions and constructionsare omitted for clarity and conciseness.

The terms and words used in the following description and claims are notlimited to the bibliographical meanings, but, are merely used by theinventor to enable a clear and consistent understanding of theinvention. Accordingly, it should be apparent to those skilled in theart that the following description of exemplary embodiments of thepresent invention is provided for illustration purpose only and not forthe purpose of limiting the invention as defined by the appended claimsand their equivalents.

It is to be understood that the singular forms “a,” “an,” and “the”include plural referents unless the context clearly dictates otherwise.Thus, for example, reference to “a component surface” includes referenceto one or more of such surfaces.

Exemplary embodiments of the present invention provide a technology foreffectively performing beam-forming under a mobile communicationenvironment in a wireless communication system. Hereinafter, exemplaryembodiments of the present invention are described using an OrthogonalFrequency Division Multiplexing (OFDM)/Orthogonal Frequency DivisionMultiple Access (OFDMA) wireless communication system as an example.However, the present invention is not limited thereto. Exemplaryembodiments of the present invention may be implemented in a wirelesscommunication system that uses a Super High Frequency (SHF) band.However, the present invention is not limited thereto. Exemplaryembodiments of the present invention are applicable to wirelesscommunications between any two nodes in either direction (i.e., wirelesscommunications from a transmitting node to a receiving node). Examplesof the nodes include, base stations, terminals, relay stations, etc. Forconvenience in explanation, exemplary embodiments of the presentinvention are described herein in the context of beam forming fortransmissions from a base station (as a transmitting node) to a terminal(as a receiving node). However, as described above, the presentinvention is not limited thereto.

FIG. 1 is a view illustrating a beam region depending on a base stationand a terminal in a wireless communication system according to anexemplary embodiment of the present invention.

Referring to FIG. 1, a beam 120 is formed when a base station 100performs beam-forming to communicate with a terminal 110. The beam 120is a region in which the terminal 110 may effectively receive abeam-formed signal. The beam 120 may have a beam width that definesboundaries of the beam 120 and that extend from the base station 100.The beam width may be a 3 dB-beam width in which the boundaries thatextend from the base station 100 correspond to locations at which thesignal strength of the beam-formed signal has decreased by 3 dB. Thebeam width may be defined in degrees. A valid distance x 130 of beam 120depends on a distance d 140 between the base station 100 and theterminal 110, and the beam width. The valid distance x 130 is a distanceof a perpendicular cross section of the beam 120. The valid distance x130 may be used to determine the validity of an operational periodbetween beam obtain procedures for the beam 120 in the case where theterminal 110 moves. The beam obtain procedure is performed periodicallyand each iteration of the beam obtain procedure is used to select andapply the beam 120 for a terminal 100. To be effective, the operationalperiod should be long enough such that a selected beam 120 continues tobe the preferred beam despite movement of the terminal 100.

For a given beam width, the valid distance x 130 is shortened as thedistance d 140 from the base station 100 is reduced. Also, for a givendistance d 140 from the base station 100, the valid distance x 130 isshortened as the beam width narrows. For example, in the case where the3 dB-beam width is 10° or 20°, when a movement velocity of the terminal110 is uniform, the valid distance x 130 of the beam 120 may becalculated depending on the distance d 140 between the base station 100and the terminal 110. In addition, when the valid distance x 130 iscalculated, a time consumed for the terminal 110 to move a distancecorresponding to the valid distance x 130 may be calculated depending ona movement velocity of the terminal 110. Table 1 shows results obtainedby calculating consumption time for moving a distance corresponding tothe valid distance x 130 depending on a movement velocity of theterminal 110, a valid distance x 130, and a distance d 140 between thebase station 100 and the terminal 110. In Table 1, it is assumed that afrequency band is 28 GHz.

TABLE 1 Time duration Distance (d) Width (x) 30 km/h 120 km/h  50 m4.3~17.6 m 0.5~2 sec 0.13~0.52 sec 100 m 8.7~35.2 m  1~4 sec   0.26~1sec

As illustrated in Table 1, even when the terminal 110 moves at a highvelocity of 120 km/h or more, hundreds of msec are consumed at theminimum for the terminal 110 to move a distance corresponding to thevalid distance x 130. Considering that a section of one frame of acommunication system is typically designed to be 10 msec or less, aconsumption time of hundreds of msec is a relatively long time. Inaddition, hundreds of msec is a relatively long time, considering are-transmission period of a Hybrid Automatic Repeat reQuest (HARQ)technique which performs retransmission by adaptively considering achannel change. That is, since a rate of change in a preferred beam dueto movement of the terminal 110 is slower than a rate of change in achannel, it is suitable that, with respect to system overhead andefficiency, a beam obtain procedure period is longer than an HARQretransmission period.

When beam-forming is performed, a beam selection error may occur. Beamselection error denotes not only a case where a beam is erroneouslyselected in a beam obtain procedure but also a case where a beamselected in the previous beam obtain procedure and a currently preferredbeam are different from each other due to movement of a terminal and/ora neighbor environment change. Here, the preferred beam denotes a beamhaving a greatest likelihood of providing a sufficient communicationquality or a beam having a greatest likelihood of providing acommunication quality greater than or equal to a threshold. Thecommunication quality may correspond to at least one of signal strength,channel quality, etc. The at least one of signal strength, channelquality, etc. may be determined using at least one of Received SignalStrength Indication (RSSI), a Signal to Interference-plus-Noise Ratio(SINR), a Signal-to-Noise Ratio (SNR), a Carrier toInterference-plus-Noise Ratio (CINR), a Carrier to Interference Ratio(CIR), a Bit Error Rate (BER), etc. When the beam selection erroroccurs, an antenna gain is reduced, thereby potentially resulting in areduced received signal strength and a deterioration in communicationquality.

The consumption time for a terminal to move a distance corresponding tothe valid distance described with reference to FIG. 1 denotes a time fora case in which a terminal moves from one boundary of the same beamregion to the opposite boundary. However, since a terminal does notalways move from one boundary of the same beam region to the oppositeboundary, the consumption time shown in Table 1 represents a maximumvalue. Therefore, in the case where the terminal moves from one beamregion to another beam region, crossing over a boundary there between,the consumption time may be shorter.

FIG. 2 is a view illustrating an example where a beam selection erroroccurs in a wireless communication system according to an exemplaryembodiment of the present invention.

Referring to FIG. 2, when a terminal 220 located near a boundary of atransmission beam-A 210 moves toward a transmission beam-B 212, a beampreferred by the terminal 220 may change before an operational period ofthe beam obtain procedure elapses, even when the velocity of theterminal 220 is low. As a result, a beam selection error may occur.Here, it is assumed that the beam obtain procedure has a longer periodthan an HARQ retransmission period, as discussed above with reference toFIG. 1.

The beam selection error may also result from neighboring environmentchanges, and not a terminal's movement. That is, even though theterminal does not leave a beam region, a preferred beam direction maychange before the beam obtain procedure period elapses due to anobstacle existing in the neighborhood, for example, an automobile, abuilding, etc. For example, as illustrated in FIG. 2, when the terminal230 remains at a fix location within a transmission beam-C 214 and anautomobile 240 enters a Line of Sight (LOS) path of the terminal 230 andthe base station 200, a beam preferred by the terminal 230 may change.This assumes that the terminal 230 may favor a Non-LOS (NLOS), that is,a direction of a beam reflected by a reflecting body and propagating, aswell as a beam of an LOS direction.

As described above, a beam selection error may occur in various cases.An intuitive and simple technique to address the beam selection error isto define a very short beam obtain procedure period. However, as thebeam obtain procedure period gets shorter, a frequency of beam obtainprocedures increases, resulting in an increased number of unnecessarybeam obtain procedures, and simultaneously, system overhead increasesdue to uplink feedback for the beam obtain procedures. In addition, dueto an increase in a frequency of beam obtain procedures, powerconsumption of the terminal may increase. Therefore, an exemplaryembodiment of the present invention proposes an alternative foreffectively addressing a beam selection error without simply shorteningthe beam obtain procedure period.

FIG. 3 is a view illustrating an example of preferred beam determinationin a wireless communication system according to an exemplary embodimentof the present invention.

Referring to FIG. 3, a base station 300 sequentially transmits aplurality of reference signals in corresponding transmission beamdirections in order to determine which beam to use to transmit toterminal 310. Here, the reference signals are signals configured in apredetermined sequence, and may be denoted by a preamble, a midamble, atraining signal, etc. The terminal 310 receives the reference signalstransmitted in the different beam directions and measures a signalstrength with respect to each reference signal. The signal strength maycorrespond to one of RSSI, SINR, SNR, CINR, CIR, BER, etc. The signalstrength may be replaced by a channel quality, etc. The terminal 310compares the measured signal strengths and then selects a transmissionbeam having a strongest signal strength. Through this process, anoptimal transmission beam is determined.

In addition, the terminal 310 may determine an optimal reception beam.In a case of determining the optimal reception beam, a reference signalshould be repeatedly transmitted a number of times corresponding to aproduct of the number of transmission beams of the base station 300 andthe number of reception beams of the terminal 310. That is, the terminal310 changes a reception beam with respect to a reference signal to whicha determined transmission beam has been applied to repeatedly receive areception beam, measures signal strengths corresponding to respectivereception beams, and then selects a reception beam having a strongestsignal strength. Through the above-described reception signal strengthmeasurement and comparison, a preferred transmission/reception beam pairmay be determined. For example, in the case of FIG. 3, an exemplaryembodiment of the present invention assumes that a transmission beam-E325 and a reception beam-B 332 have been selected as a preferredtransmission/reception beam pair having the strongest signal strength.

When the transmission beam-E 325 and the reception beam-B 332 areselected as optimal transmission/reception beams, the terminal 310 feedsback information to the to the base station 300 to inform the basestation 300 that the transmission beam-E 325 is a preferred transmissionbeam. After that, in the case where the preferred transmission/receptionbeam pair changes, that is, where a beam selection error occurs duringdata transmission or before the next beam obtain procedure is performed,the terminal 310 should reselect a preferred transmission/reception beampair, and inform the base station of the reselected transmission beam.However, the terminal 310 is not certain whether the base station 300will immediately apply the reselected transmission beam, and reselectionof a reception beam corresponding to the reselected transmission beam isstill required. Therefore, to address the uncertainty of whether thereselected transmission beam is applied and the necessity of thereception beam reselection, an additional control signal exchange isrequired. Therefore, the base station 300 and the terminal 310 mayclearly determine whether to apply the reselected transmission beam andreselect a reception beam corresponding to the reselected transmissionbeam through the exchange of the additional control signal.

Additionally, a technique is provided herein for reducing a signalingoverhead by removing the exchange of the additional control signalaccording to an exemplary embodiment of the present invention. Analternative to removing the exchange of the additional control signal isto reselect a transmission beam in the range in which preference of areception beam is maintained. Generally, a reception beam of theterminal 310 has a wider beam width than a transmission beam of the basestation 300. Therefore, with respect to one preferred reception beam, aplurality of preferred transmission beams may exist. In other words, arelationship between a reception beam and a transmission beam may be 1:plurality. For example, in the case of FIG. 3, an exemplary embodimentof the present invention assumes that a correspondence relationshipbetween a reception beam and a transmission beam is illustrated in Table2.

TABLE 2 Tx beam RX beam A A B F C B D A E B F B G C H C

As shown in Table 2, the terminal 310 prefers a transmission beam-A 321and a transmission beam-D 324 with respect to a reception beam-A 331,prefers a transmission beam-C 323, a transmission beam-E 325 and atransmission beam-F 326 with respect to a reception beam-B 332, prefersa transmission beam-G 327 and transmission beam-H 328 with respect to areception beam-C 333, and prefers a transmission beam-B 322 with respectto a reception beam-F 336. Herein, the terminal 310 does not prefer anytransmission beam with respect to reception beam-D 334 and receptionbeam-E 335.

During a beam obtain procedure, the terminal 310 determines a receptionbeam having a strongest signal strength for each transmission beam togenerate a transmission/reception beam pair list. For example, thetransmission/reception beam pair list may be the list shown in Table 2.Accordingly, when the terminal 310 then determines that a beam selectionerror occurs, the terminal 310 selects a beam pair having a strongestsignal strength from the transmission/reception beam pairs including thereception beam currently in use. In addition, the terminal 310 feedsback information to the base station 300 to inform the base station 300of a transmission beam included in the selected beam pair as reselectedtransmission beam information. In this case, since the reception beamdoes not change, it does not matter in the operation of the terminal 310whether the reselected transmission beam is applied because reselectionof the reception beam is not required.

A description is made below using a specific example with reference toFIG. 3. An optimal transmission/reception beam pair determined via abeam obtain procedure in FIG. 3 is a transmission beam-E 325 and areception beam-B 332. At this point, during the beam obtain procedure,the terminal 310 generates and stores a transmission/reception beam pairlist. After that, the terminal 310 detects the occurrence of a beamselection error using reference signals transmitted for a differentpurpose or a beam obtain procedure of a different terminal. That is, theterminal 310 may use signals transmitted from the base station 300 for apurpose other than the beam obtain procedure as the reference signals ora the terminal may use reference signals intended for a beam obtainprocedure for a terminal other than the terminal 310. In the case ofusing reference signals for a different purpose or the beam obtainprocedure for a different terminal, the terminal 310 may receive atleast one reference signal every frame.

An exemplary process of detecting a beam selection error is describedbelow in more detail. The terminal 310 newly generates atransmission/reception beam pair list using the reference signals, andsearches for a beam pair having a strongest signal strength amongtransmission/reception beam pairs including the reception beam-B 323.When a beam pair is found, which includes the reception beam-B 323 andhas the strongest signal strength, that is different from thetransmission/reception beam pair determined via the beam obtainprocedure, the terminal 310 determines that a beam selection erroroccurs. In this case, the terminal 310 feeds back information to thebase station 300 representing a transmission beam included in the beampair, which includes the reception beam-B 323, that is found to have thestrongest signal strength, as information for a reselected transmissionbeam. Through this, the terminal 310 may receive a signal of the basestation 300 without needing to determine whether a reselectedtransmission beam has been applied and without having to reselect areception beam.

For another example, the terminal 310 does not newly generate atransmission/reception beam pair list, but may still determine whetherthe beam selection error occurs. Specifically, in a case where apreferred reception beam is applied, the terminal 310 selects atransmission beam that maximizes reception signal strength, anddetermines whether the transmission beam is the same as the transmissionbeam determined via the beam obtain procedure, thereby determiningwhether the beam selection error occurs. Here, when selecting thetransmission beam that maximizes the reception signal strength, theterminal 310 may select any transmission beam. Alternatively, whenselecting the transmission beam that maximizes the reception signalstrength, the terminal 310 may limit its selection to those transmissionbeams that correspond to the preferred reception beam in thetransmission/reception beam pair list. In addition, in a case where thepreferred reception beam is applied, the terminal 310 feeds backinformation to the base station 300 representing the transmission beamthat maximizes reception signal strength.

During the process of determining a beam selection error, when thetransmission beam that maximizes reception signal strength is differentfrom the transmission beam determined via the beam obtain procedure, theterminal 310 determines the beam selection error. However, according toanother exemplary embodiment of the present invention, a firstadditional condition may need to be met for the terminal 310 todetermine that a beam selection error occurs. The first additionalcondition is that the signal strength of the transmission beamdetermined via the beam obtain procedure is less than a first threshold.For example, when the signal strength of the transmission beamdetermined via the beam obtain procedure is greater than or equal to thefirst threshold, despite the reselected transmission beam that maximizesthe reception signal strength being different from the transmission beamdetermined via the beam obtain procedure, the terminal 310 may determinethat a beam selection error has not occurred. In another example, whenthe signal strength of the transmission beam determined via the beamobtain procedure is less than the first threshold, and the reselectedtransmission beam is different from the transmission beam determined viathe beam obtain procedure, the terminal 310 may determine that a beamselection error has occurred.

Instead of or in addition to the first additional condition describedabove, a second condition may need to be met for the terminal 310 todetermine that a beam selection error occurs. The second additionalcondition is that the difference between signal strength of thereselected transmission beam and the signal strength of the transmissionbeam determined via the beam obtain procedure is greater than a secondthreshold. For example, when the difference between the signal strengthof the reselected transmission beam and the signal strength of thetransmission beam determined via the beam obtain procedure is less thanor equal to the second threshold, despite the reselected transmissionbeam that maximizes the reception signal strength being different fromthe transmission beam determined via the beam obtain procedure, theterminal 310 may determine that a beam selection error has not occurred.In another example, when the difference between signal strength of thereselected transmission beam and the signal strength of the transmissionbeam determined via the beam obtain procedure is greater than the secondthreshold, and the reselected transmission beam is different from thetransmission beam determined via the beam obtain procedure, the terminal310 may determine that a beam selection error has occurred.

As described above, the base station and the terminal according to anexemplary embodiment of the present invention perform a beam obtainprocedure using a reference signal according to a predetermined period,and simultaneously, may address a beam change that may occur temporarilyor irregularly via a beam change procedure performed when a beamselection error occurs.

A case where the above-described beam obtain procedure and beam changeprocedure are performed according to an exemplary embodiment of thepresent invention is described below. For example, a beam changeprocedure according to an exemplary embodiment of the present inventionmay be performed when a beam selection error occurs or when an HARQinitial transmission or retransmission fails and simultaneously a beamselection error occurs. FIG. 4 illustrates an exemplary embodiment thatincludes the HARQ transmission failure and the beam selection erroroccurrence as a condition for performing the beam change procedure, andFIG. 5 illustrates an exemplary embodiment that includes a beamselection error occurrence as a condition for performing the beam changeprocedure.

FIG. 4 is a view illustrating signaling for a beam obtain procedure anda beam change procedure in a wireless communication system according toan exemplary embodiment of the present invention.

Referring to FIG. 4, a beam obtain procedure is performed in steps 401and 403. In step 401, a base station 400 sequentially transmitsreference signals that apply different transmission beams, and aterminal 410 applies different reception beams to receive the referencesignals. That is, the base station 400 repeatedly transmits a pluralityof reference signals while changing beam directions by the number oftransmission/reception beams. Herein, there are alternativeimplementations for step 403 of the beam obtain procedure.

In a first alternative for step 403, the terminal 410 selects apreferred beam using the reference signals and feeds back information ofthe preferred beam. In other words, the terminal 410 measures the signalstrength of each reference signal transmitted by the base station 400,selects a beam direction having a strongest signal strength from themeasured signal strengths, and feeds back an index of the selected beamor an index of the selected reference signal to the base station 400.

In a second alternative for step 403, instead of only selecting thepreferred beam having the strongest signal strength, the terminal 410may select the best N beams and feed back an index of the selected Nbeams or an index of the selected N reference signals to the basestation 400. The terminal 410 may also include at least one of thecorresponding measurements of the N reference signals, an indication ofa preferred beam or reference signal, a ranking of the selected N beamsand a ranking of the selected N signals, etc. In this case, the basestation 400 may use the best of the N beams. Here, N may be a fixednumber or N may be the number of reference signals that have a signalstrength that is greater than a threshold.

Herein, the above alternative implementations for step 403 of the beamobtain procedure will correspond to respective alternativeimplementations for the beam change procedure, which are describedfurther below. In addition, the use of signal strength may be replacedby channel quality, etc. Steps 401 and 403 may be repeatedly performedaccording to a predetermined period. At this point, it is preferablethat a period of a beam obtain procedure is set much longer than an HARQretransmission period as described above.

The base station 400 initially transmits data to the terminal 410 instep 405. Here, an HARQ technique is applied to the data. At this point,for convenience in description, it is assumed that the terminal 410fails to decode the initially transmitted data. The terminal 410 thathas failed to decode the data transmits Non ACKnowledge (NACK) to thebase station 400 in step 407. According to the HARQ technique, in thecase where the NACK is transmitted, the base station 400 retransmits thedata, and the terminal 410 improves reception performance via couplingof an initially transmitted signal and a retransmitted signal.Typically, the cause of the decoding failure is an erroneous selectionof a signal modulation and encoding caused by a difference between anexpected channel and an actual channel. Also, as described above withreference to FIG. 2, the cause of the decoding failure may be a beamselection error. Therefore, the base station 400 and the terminal 410according to an exemplary embodiment of the present invention determinewhether a beam selection error occurs before retransmission, and performa beam change procedure when the beam selection error occurs.

The beam change procedure is a procedure for modifying a beam directionin a relatively simple manner as compared to the beam obtain procedure.During the beam change procedure, replacement or addition of a beam ismade. Even when a beam obtain procedure period does not elapse, areference signal may be transmitted every frame for a different purposeor a different terminal. Therefore, the terminal 410 may determine apreferred beam every frame using the reference signals for the differentpurpose or the different terminal, and determine whether the beamselection error occurs. The base station 400 may then replace or add abeam direction for the terminal 410. The particular implementation ofthe beam change procedure corresponds to respective implementations ofthe beam obtain procedure. For convenience in explanation, it is assumedthat the beam selection error has occurred.

The terminal 410, after determining that the beam selection error hasoccurred, transmits a beam change request to the base station 400 instep 409. The information included in the beam change request variesdepending of the particular exemplary embodiment implemented. When thefirst alternative implementation of step 403 is performed, the beamchange request may include at least one of information of a reselectedtransmission beam and information designating one of a change and anaddition of a beam. Accordingly, the base station 400 may replace or adda beam direction for the terminal 410. As illustrated in FIG. 4, abeam-B may be replaced by a beam-C, or the beam-C may be added.

When the second alternative implementation of step 403 is performed, thebeam change request is only an indication that a beam selection errorhas occurred. The indication that the beam selection error has occurredmay be communicated using a small amount of information, such as onebit. In this case, the indication that the beam selection error hasoccurred may be included with another message, such as the NACK message407, thereby reducing an amount of signaling. Also in this case, thebase station 400 upon receiving the indication that the beam selectionerror has occurred may determine whether to replace or add a beamdirection for the terminal 410 based on the measurements of the Nreference signals received in step 403. As illustrated in FIG. 4, abeam-B may be replaced by a beam-C, or the beam-C may be added.

After performing the beam change procedure, the base station 400retransmits data to the terminal 410 in step 411. At this point, anexemplary embodiment of the present invention assumes that the terminal410 succeeds in decoding the data via coupling of an initiallytransmitted signal and a retransmitted signal. The terminal 410 that hassucceeded in decoding the data transmits an ACKnowledge (ACK) to thebase station 400 in step 413.

After that, when a period of the beam obtain procedure elapses, the basestation 400 and the terminal 410 perform the beam obtain procedure. Thatis, the base station 400 sequentially transmits reference signals thatapply respective transmission/reception beams, and the terminal 410applies different reception beams to receive the reference signals instep 415. In addition, the terminal 410 selects a preferred beam orpreferred N beams using the reference signals, and feeds back thepreferred beam or the preferred N beams with their measurements and/orpriority in step 417.

In the exemplary embodiment described with reference to FIG. 4, the beamchange procedure is performed in the case where an HARQ retransmissionis requested between beam obtain procedure periods and a beam selectionerror occurs. However, the exemplary embodiment illustrated in FIG. 4cannot resolve a beam selection error that has occurred during aninitial transmission. Therefore, according to another exemplaryembodiment of the present invention, the beam change procedure may beperformed in the case where a beam selection error occurs regardless ofHARQ retransmission. In this case, improvement of initial transmissionperformance of data is expected. Another exemplary embodiment of thepresent invention is illustrated in FIG. 5.

FIG. 5 is a view illustrating signaling for a beam obtain procedure anda beam change procedure in a wireless communication system according toan exemplary embodiment of the present invention.

Referring to FIG. 5, a beam obtain procedure is performed in steps 501and 503. In step 501, a base station 500 sequentially transmitsreference signals that apply respective transmission/reception beams,and a terminal 510 applies different reception beams to receive thereference signals in step 501. That is, the base station 500 repeatedlytransmits a plurality of reference signals while changing beamdirections by the number of transmission/reception beams. Herein, thereare alternative implementations for step 503 of the beam obtainprocedure.

In a first alternative for step 503, the terminal 510 selects apreferred beam using the reference signals and feeds back information ofthe preferred beam in step 503. In other words, the terminal 510measures a signal strength of each reference signal transmitted by thebase station 500, selects a beam direction having a strongest signalstrength from the measured signal strengths, and feeds back an index ofthe selected beam or an index of the selected reference signal to thebase station 500.

In a second alternative for step 503, instead of only selecting thepreferred beam having the strongest signal strength, the terminal 510may select the best N beams and feed back an index of the selected Nbeams or an index of the selected N reference signals to the basestation 500. The terminal 510 may also include at least one of thecorresponding measurements of the N reference signals, an indication ofa preferred beam or reference signal, a ranking of the selected N beamsand a ranking of the selected N signals, etc. In this case, the basestation 500 may use the best of the N beams. Here, N may be a fixednumber or N may be the number of reference signals that have a signalstrength that is greater than a threshold.

Herein, the above alternative implementations for step 503 of the beamobtain procedure will correspond to respective alternativeimplementations for the beam change procedure, which are describedfurther below. In addition, the signal strength may be replaced bychannel quality, etc. Steps 501 and 503 may be repeatedly performedaccording to a predetermined period. At this point, it is preferablethat a period of a beam obtain procedure is set much longer than an HARQretransmission period as described above.

After the beam obtain procedure, but before a beam obtain procedureperiod elapses, the terminal 510 may detect the occurrence of a beamselection error. That is, a reference signal may be transmitted by thebase station 500 every frame for a different purpose or a differentterminal. Therefore, the terminal 510 may determine a preferred beamevery frame using the reference signals for the different purpose or thedifferent terminal, and determine whether the beam selection erroroccurs. The base station 500 may then replace or add a beam directionfor the terminal 510. The particular implementation of the beam changeprocedure corresponds to respective implementations of the beam obtainprocedure. For convenience in explanation, it is assumed that the beamselection error has occurred.

The terminal 510, after determining that the beam selection error hasoccurred, transmits a beam change request to the base station 500 instep 505. The information included in the beam change request variesdepending of the particular exemplary embodiment implemented. When thefirst alternative implementation of step 503 is performed, the beamchange request may include at least one of information of a reselectedtransmission beam and information designating one of a change and anaddition of a beam. Accordingly, the base station 500 replaces or adds abeam direction for the terminal 510. As illustrated in FIG. 5, a beam-Bmay be replaced by a beam-C, or the beam-C may be added.

When the second alternative implementation of step 503 is performed, thebeam change request is only an indication that a beam selection errorhas occurred. The indication that the beam selection error has occurredmay be communicated using a small amount of information, such as onebit. In this case, the indication that the beam selection error hasoccurred may be included with another message so as to reduce an amountof signaling. Also in this case, the base station 500 upon receiving theindication that the beam selection error has occurred may determinewhether to replace or add a beam direction for the terminal 510 based onthe measurements of the N reference signals received in step 503. Asillustrated in FIG. 5, a beam-B may be replaced by a beam-C, or thebeam-C may be added.

After that, when a period of the beam obtain procedure elapses, the basestation 500 and the terminal 510 perform a beam obtain procedure. Thatis, the base station 500 sequentially transmits reference signals thatapply respective transmission/reception beams, and the terminal 510applies different reception beams to receive the reference signals instep 507. The terminal 510 selects a preferred beam or preferred N beamsusing the reference signals and feeds back the preferred beam or thepreferred N beams with their measurements and/or priority in step 509.

Hereinafter, operations and constructions of a terminal and a basestation that perform beam-forming are described in more detail withreference to the drawings.

FIG. 6 is a flowchart illustrating a procedure for operating a terminalin a wireless communication system according to an exemplary embodimentof the present invention.

Referring to FIG. 6, the terminal determines a preferredtransmission/reception beam or preferred N transmission beams for thepreferred reception beam via a beam obtain procedure in step 601. Thebeam obtain procedure is periodically performed according to apredetermined time interval. More specifically, the terminalsequentially receives reference signals that apply differenttransmission beams from the base station, and applies reference signalsto which the same transmission signal has been applied to differentreception beams. Through this, the terminal determines a preferredtransmission/reception beam pair or preferred N transmission beams forthe preferred reception beam. Accordingly, the terminal applies thepreferred reception beam to a downlink signal from the base station.

After determining the preferred transmission/reception beam pair orpreferred N transmission beams for the preferred reception beam, theterminal proceeds to step 603 to feed back preferred transmission beaminformation to the base station. Since a reception beam is used by theterminal, the information indicating the preferred reception beam is notfed back. Herein, there are alternative implementations for feeding backthe information indicating the preferred transmission beam. In a firstalternative for feeding back the information indicating the preferredtransmission beam, the information indicating the preferred transmissionbeam may include an index of the preferred beam or an index of thereference signal corresponding to the preferred beam. In a secondalternative for feeding back the information indicating the preferredtransmission beam, the information indicating the preferred Ntransmission beams may include an index of the preferred N beams or anindex of the N reference signals corresponding to the preferred N beams.The information indicating the preferred N transmission beams may alsoinclude at least one of the corresponding measurements of the Nreference signals, an indication of a preferred beam or referencesignal, a ranking of the selected N beams and a ranking of the selectedN signals, etc. In this case, the base station may use the best of the Nbeams as the preferred transmission beam. Here, N may be a fixed numberor N may be the number of reference signals that have a signal strengththat is greater than a threshold. Herein, the above alternativeimplementations for feeding back the information indicating thepreferred transmission beam will correspond to respective alternativeimplementations for requesting a transmission beam change, which aredescribed further below.

The terminal proceeds to step 605 to generate a transmission/receptionbeam pair list. The transmission/reception beam pair list representsresults of selecting a reception beam maximizing received signalstrength with respect to each transmission beam. For example, thetransmission/reception beam pair list may be configured as shown inTable 2. That is, since a reception beam of the terminal typically has awider beam than a transmission beam of the base station, a relationshipbetween the reception beam and the transmission beam may be 1:plurality. In other words, with respect to one reception beam, aplurality of preferred transmission beams may exist. Step 605 may beperformed prior to step 603 or simultaneously with steps 601 or 603.

The terminal proceeds to step 607 to determine whether a beam change isrequired. A condition under which the beam change is performed may bedifferent depending on a specific exemplary embodiment implemented. Forexample, if the exemplary embodiment described above with reference toFIG. 4 is implemented, that is, in the case where decoding of data towhich the HARQ technique has been applied fails, and simultaneously, abeam selection error occurs, the terminal may determine that the beamchange is required. For another example, if the exemplary embodimentdescribed with reference to FIG. 5 is implemented, that is, in the casewhere only the beam selection error occurs, the terminal may determinethat the beam change is required.

To determine whether the beam selection error occurs, the terminal mayuse reference signals other than reference signals used for the beamobtain procedure of the terminal. For example, the terminal newlygenerates a transmission/reception beam pair list using referencesignals other than the reference signals used for the beam obtainprocedure of the terminal, searches for a beam pair having a strongestsignal strength among transmission/reception beam pairs including thepreferred reception beam selected in advance, and when a beam pairhaving the strongest signal strength is found that is different from thepreferred transmission/reception beam pair determined via the beamobtain procedure, determines that the beam selection error occurs. Foranother example, in the case where a preferred reception beam isapplied, the terminal determines a transmission beam maximizing receivedsignal strength again, and when the transmission beam is different froma preferred transmission beam determined via the beam obtain procedure,determines that the beam selection error occurs. In determining theoccurrence of the beam selection error, though the transmission beammaximizing the received signal strength is different from the preferredtransmission beam determined via the beam obtain procedure, when signalstrength of a case that applies the transmission beam determined via thebeam obtain procedure is greater than or equal to a threshold, theterminal may determine that a beam selection error has not occurred.That is, a condition that signal strength in a case of applying atransmission beam determined via the beam obtain procedure is less thanthe threshold may be added as a condition under which the beam selectionerror occurs.

When the beam change is required, the terminal proceeds to step 609 totransmit a beam change request to the base station. The informationincluded in the beam change request varies depending of the particularexemplary embodiment implemented. When the first alternative for feedingback the information indicating the preferred transmission beam isimplemented, the beam change request may include at least one ofinformation of a reselected preferred transmission beam and informationdesignating one of a change and an addition of a beam. At this point,the terminal reselects a transmission beam in the range where apreferred reception beam is the same. That is, the terminal reselects atransmission beam maximizing received signal strength as a preferredtransmission beam while the preferred reception beam determined in step601 is applied.

When the second alternative for feeding back the information indicatingthe preferred transmission beam is implemented, the beam change requestis only an indication that a beam selection error has occurred. Theindication that the beam selection error has occurred may becommunicated using a small amount of information, such as one bit. Inthis case, the indication that the beam selection error has occurred maybe included with another message, such as the NACK message, therebyreducing an amount of signaling.

The terminal proceeds to step 611 to determine whether a period of abeam obtain procedure elapses. That is, the beam obtain procedure isperiodically performed according to a predetermined time interval. Whenthe period of the beam obtain procedure does not elapse, the terminalreturns to step 607. When the period of the beam obtain procedureelapses, the terminal returns to step 601 to perform the beam obtainprocedure again.

FIG. 7 is a flowchart illustrating a procedure for operating a basestation in a wireless communication system according to an exemplaryembodiment of the present invention.

Referring to FIG. 7, the base station determines a preferredtransmission beam of a terminal via a beam obtain procedure in step 701.The beam obtain procedure is periodically performed according to apredetermined time interval. Specifically, the base station sequentiallytransmits reference signals that apply different transmission beams, andreceives preferred transmission beam information from the terminal. Inan exemplary implementation, the preferred transmission beam informationmay include an index of a beam preferred by the terminal or an index ofa reference signal corresponding to a beam preferred by the terminal.Accordingly, the base station identifies the preferred transmission beambased on the preferred transmission beam information and applies thepreferred transmission beam to a downlink signal toward the terminal.

In another exemplary implementation, the preferred transmission beaminformation may include an index of N beams preferred by the terminal oran index of N reference signals corresponding to beams preferred by theterminal. The preferred transmission beam information may also includeat least one of corresponding measurements of the N reference signalstaken by the terminal, an indication of a beam or reference signalpreferred by the terminal, a ranking of the N beams by the terminal, aranking of the selected N signals by the terminal, etc. Here, N may be afixed number or N may be the number of reference signals having a signalstrength as measured by the terminal that is greater than a threshold.Accordingly, the base station identifies the preferred transmission beambased on the preferred transmission beam information and applies thepreferred transmission beam to a downlink signal toward the terminal.

The base station proceeds to step 703 to determine whether a beam changerequest is received from the terminal. In one embodiment, the beamchange request is a signal informing that the transmission beampreferred by the terminal changes, and may include at least one ofinformation of a reselected preferred transmission beam and informationdesignating change/addition of a beam. In another embodiment, the beamchange request is an indication that a beam selection error hasoccurred. The indication that the beam selection error has occurred maybe communicated using a small amount of information, such as one bit. Inthis case, the indication that the beam selection error has occurred maybe included with another message, such as a NACK message, therebyreducing an amount of signaling.

Here, whether the base station is restrained in designatingchange/addition of a beam of the terminal may change depending on aspecific embodiment implemented. Even though the base station isrestrained in designating change/addition of a beam of the terminal,when addition of a beam is impossible due to resource shortage of thebase station, only beam change is allowed. In addition, in the casewhere information designating change/addition of the beam is notincluded, the base station may determine that addition of the beam isalways possible with priority. When the beam change request is notreceived, the base station proceeds to step 711.

In contrast, when the beam change request is received, the base stationproceeds to step 705 to determine whether beam addition is possible.Whether the beam addition is possible may be determined based on whetherRadio Frequency (RF) resources of the base station are available. Thatis, to enable addition of the beam, the base station should have aplurality of RF chains, and should be able to additionally allocate atleast one RF chain to a downlink signal toward the terminal. In additionor alternatively, the base station may determine whether beam additionis possible based on the preferred transmission beam information fedback from the terminal during the beam obtain procedure. If the additionof the beam is possible, the base station proceeds to step 707 to add areselected preferred transmission beam to the preferred transmissionbeam determined in step 701. Here, the reselected preferred transmissionbeam may be the beam included in the beam change request or may be abeam determined by the base station based on the preferred transmissionbeam information fed back from the terminal during the beam obtainprocedure. In contrast, if the addition of the beam is not possible, thebase station proceeds to step 709 to replace the preferred transmissionbeam determined in step 701 with the reselected preferred transmissionbeam. Once again, the reselected preferred transmission beam may be thebeam included in the beam change request or may be a beam determined bythe base station based on the preferred transmission beam informationfed back from the terminal during the beam obtain procedure.

The base station proceeds to step 711 to determine whether a period of abeam obtain procedure elapses. That is, the beam obtain procedure isperiodically performed according to a predetermined time interval. Whenthe period of the beam obtain procedure does not elapse, the basestation returns to step 703. When the period of the beam obtainprocedure elapses, the base station returns to step 701 to perform thebeam obtain procedure again.

FIG. 8 is a block diagram illustrating a terminal in a wirelesscommunication system according to an exemplary embodiment of the presentinvention.

Referring to FIG. 8, the terminal includes an antenna array 810, abeam-forming unit 820, a reception RF chain 830, a modem 840, atransmitter 850, and a controller 860. According to exemplaryembodiments of the present invention, the terminal may includeadditional and/or different components. Similarly, the functionality oftwo or more components may be integrated into a single component.

The antenna array 810 is an aggregate of a plurality of antennas, andincludes a plurality of array elements. The beam-forming unit 820performs reception beam-forming with respect to a signal received via aplurality of antennas forming the antenna array 810. For example, thebeam-forming unit 820 includes a plurality of amplifiers, a plurality ofphase shifters, and a signal adder. That is, the beam-forming unit 820performs reception beam-forming by controlling and adding the phase ofsignals received via each of the plurality of antennas. The reception RFchain 830 converts an analog reception signal in an RF band into abaseband digital signal. For example, the reception RF chain 830 mayinclude an amplifier, a mixer, an oscillator, an Analog-to-DigitalConverter (ADC), and a filter. The antenna array 810, beam-forming unit820, and beam-forming unit 820 may collectively be referred to as areceiver. Also, the receiver may vary depending on the particularimplementation of the receiver. For example, the terminal may notinclude the beam-forming unit 820 and beam-forming unit 820.

The modem 840 performs a conversion function between a baseband signaland a bit line according to a physical layer standard of a system. Forexample, in an OFDM scheme, during data transmission, the modem 840generates complex symbols by encoding and modulating a transmission bitline, maps the complex symbols to subcarriers, and then configures OFDMsymbols via Inverse Fast Fourier Transform (IFFT) and Cyclic Prefix (CP)insertion. In addition, during data reception, the modem 840 dividesbaseband signals provided from the reception RF chain 830 on an OFDMsymbol basis, recovers signals mapped to subcarriers by performing FastFourier Transform (FFT), and then recovers a reception bit line throughdemodulation and decoding. The transmitter 850 converts a transmissionsignal provided from the modem 840 into an RF signal to transmit thesame to a base station. Though not shown specifically, the transmitter850 includes a transmission RF chain, an antenna, etc.

The controller 860 controls an overall operation of the terminal. Thecontroller 860 may perform or control to perform any of the operationsdescribed herein as being performed by the terminal. For example, thecontroller 860 generates transmission traffic and a message to providethe same to the modem 840, and reads a reception traffic packet and amessage provided from the modem 840. More particularly, the controller860 controls to perform the beam obtain procedure and the beam changeprocedure. An operation of the controller 860 for the beam obtainprocedure and the beam change procedure is described below.

For the beam obtain procedure, the controller 860 sequentially receivesreference signals that apply different transmission beams from the basestation, and controls the beam-forming unit 820 to apply referencesignals to which the same transmission beam has been applied todifferent reception beams. In addition, the controller 860 determines atransmission beam of the base station and a reception beam of theterminal that maximize received signal strength, that is, a preferredtransmission/reception beam pair. Accordingly, the controller 860generates preferred transmission beam information, and transmits thepreferred transmission beam information to the base station via themodem 840 and the transmitter 850. After that, the controller 860controls the beam-forming unit 820 to perform reception beam-formingusing the preferred reception beam. The beam obtain procedure isperiodically performed according to a predetermined time interval.

For the beam change procedure, after the beam obtain procedure, thecontroller 860 generates a transmission/reception beam pair list. Inaddition, the controller 860 determines whether a beam change isrequired before a period of the beam obtain procedure elapses. Acondition under which the beam change is performed may change dependingon a specific embodiment. For example, the condition includes a beamselection error occurrence, and may further include a decoding failureof data to which the HARQ technique has been applied. To determinewhether the beam selection error occurs, the controller 860 may usedifferent reference signals excluding a reference signal for the beamobtain procedure of the terminal. When the beam change is required, thecontroller 860 reselects a preferred transmission beam, and transmits abeam change request to inform the base station of the reselectedpreferred transmission beam. The beam change request may include atleast one of information of the reselected preferred transmission beamand information designating change/addition of a beam. Alternatively,the beam change request may be an indication that the beam selectionerror has occurred and may be communicated using a small amount ofinformation, such as one bit. In this case, the indication that the beamselection error has occurred may be included with another message, suchas a NACK message, thereby reducing an amount of signaling. At thispoint, the controller 860 maintains the reception beam.

FIG. 9 is a block diagram illustrating a base station in a wirelesscommunication system according to an exemplary embodiment of the presentinvention.

Referring to FIG. 9, the base station includes a modem 910, a receiver920, a plurality of transmission RF chains 930-1 to 930-N, abeam-forming unit 940, an antenna array 950, and a controller 960.According to exemplary embodiments of the present invention, the basestation may include additional and/or different components. Similarly,the functionality of two or more components may be integrated into asingle component.

The modem 910 performs a conversion function between a baseband signaland a bit line according to a physical layer standard of a system. Forexample, in an OFDM scheme, during data transmission, the modem 910generates complex symbols by encoding and modulating a transmission bitline, maps the complex symbols to subcarriers, and then configures OFDMsymbols via IFFT and CP insertion. In addition, during data reception,the modem 910 divides baseband signals on an OFDM symbol basis, recoverssignals mapped to subcarriers by performing FFT, and then recovers areception bit line through demodulation and decoding. The receiver 920converts an RF signal received from a terminal into a baseband digitalsignal. Specifically, though not shown, the receiver 920 includes anantenna, reception RF chain, etc.

The plurality of transmission RF chains 930-1 to 930-N convert basebanddigital signal streams provided from the modem 910 to an RF analogsignal. For example, each of the plurality of transmission RF chains930-1 to 930-N may include an amplifier, a mixer, an oscillator, aDigital-to-Analog Converter (DAC), and a filter. The base station maysimultaneously form transmission beams by the number of the plurality oftransmission RF chains 930-1 to 930-N.

The beam-forming unit 940 performs beam-forming on transmission signalsprovided from the plurality of transmission RF chains 930-1 to 930-N.For example, the beam-forming unit 940 includes a plurality of phaseshifters, a plurality of amplifiers, and a signal adder. That is, thebeam-forming unit 940 divides a transmission signal provided from eachof the plurality of transmission RF chains 930-1 to 930-N by the numberof the plurality of antennas included in the antenna array 950, andcontrols the phase of each divided signal. In addition, the beam-formingunit 940 adds signals to be transmitted via the same antenna. Theantenna array 950 is an aggregate of a plurality of antennas, includes aplurality of array elements, and radiates signals provided from thebeam-forming unit 940 to an RF channel. The plurality of transmission RFchains 930-1 to 930-N, beam-forming unit 940, and antenna array 950 maycollectively be referred to as a transmitter.

The controller 960 controls an overall operation of the base station.The controller 960 may perform or control to perform any of theoperations described herein as being performed by the base station. Forexample, the controller 960 generates a transmission traffic packet anda message to provide the same to the modem 910, and reads a receptiontraffic packet and a message provided from the modem 910. Moreparticularly, the controller 960 controls to perform a beam obtainprocedure and a beam change procedure. An operation of the controller960 for the beam obtain procedure and the beam change procedure isdescribed below.

For the beam obtain procedure, the controller 960 controls the modem 910to repeatedly transmit reference signals, and controls the beam-formingunit 940 to apply different transmission beams to the reference signals.In addition, the controller 960 determines a transmission beam preferredby a terminal based on preferred transmission beam information receivedvia the receiver 920. After that, the controller 960 controls thebeam-forming unit 940 to apply a transmission beam preferred by theterminal to a downlink signal toward the terminal The beam obtainprocedure is periodically performed according to a predetermined timeinterval.

For the beam change procedure, when a beam change request is receivedfrom the terminal via the receiver 920, the controller 960 determineswhether beam addition is possible. To enable the addition of the beam,at least one of the plurality of transmission RF chains 930-1 to 930-Nshould be able to be additionally allocated to a downlink signal towardthe terminal. In addition or alternatively, controller 960 may determinewhether beam addition is possible based on the preferred transmissionbeam information received via the receiver 920 during the beam obtainprocedure. When the addition of the beam is possible, the controller 960adds a reselected preferred transmission beam included in the beamchange request to the transmission beam preferred by the terminal. Here,the reselected preferred transmission beam may be the beam included inthe beam change request or may be a beam determined by the controller960 based on the preferred transmission beam information received viathe receiver 920 during the beam obtain procedure. In contrast, when theaddition of the beam is not possible, the controller 960 replaces thetransmission beam preferred by the terminal by the reselected preferredtransmission beam included in the beam change request. Once again, thereselected preferred transmission beam may be the beam included in thebeam change request or may be a beam determined by the controller 960based on the preferred transmission beam information received via thereceiver 920 during the beam obtain procedure.

Exemplary embodiments of the present invention are described herein inthe context of the base station using a transmission beam and theterminal using a reception beam. However, the present invention is notlimited thereto. Exemplary embodiments of the present invention areequally applicable to the case where the terminal does not utilize areception beam. In this case, instead of selecting beams pairs,transmission beams alone are selected.

Exemplary embodiments of the present invention may swiftly deal withtransient or temporary beam mismatch phenomenon by additionallyperforming a beam change procedure corresponding to a beam selectionerror that occur before a next iteration of a beam obtain procedure thatis periodically performed in a wireless communication system.

While the invention has been shown and described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims and their equivalents.

What is claimed is:
 1. A method for operating a receiving node in awireless communication system, the method comprising: determining afirst transmission beam as a preferred transmission beam using aplurality of reference signals transmitted by a transmitting node;generating preferred transmission beam information; transmitting thepreferred transmission beam information to the transmitting node;receiving transmissions from the transmitting node via the firsttransmission beam; determining whether a change of a transmission beamis necessary; and if the change of the transmission beam is determinedto be necessary, generating a beam change request, and transmitting thebeam change request to the transmitting node.
 2. The method of claim 1,wherein the determining of whether the change of the transmission beamis necessary comprises: trying to decode data to which a HybridAutomatic Repeat reQuest (HARQ) technique has been applied; if thedecoding fails, determining whether the first transmission beam is atransmission beam maximizing received signal strength; and if the firsttransmission beam is not the transmission beam maximizing the receivedsignal strength, determining that the change of the transmission beam isnecessary.
 3. The method of claim 1, wherein the determining of whetherthe change of the transmission beam is necessary comprises: determiningwhether the first transmission beam is a transmission beam maximizingreceived signal strength; and if the first transmission beam is not thetransmission beam maximizing the received signal strength, determiningthat the change of the transmission beam is necessary.
 4. The method ofclaim 1, wherein a condition under which the change of the transmissionbeam is necessary comprises at least one of a mismatch between the firsttransmission beam and a transmission beam maximizing received signalstrength, a failure in decoding data, and a condition in which receivedsignal strength is less than a threshold if applying the firsttransmission beam.
 5. The method of claim 1, wherein the determining ofwhether the change of the transmission beam is necessary comprisesdetermining whether the first transmission beam is a transmission beammaximizing received signal strength.
 6. The method of claim 1, whereinthe determining of the first transmission beam as the preferredtransmission beam using the plurality of reference signals transmittedby the transmitting node comprises determining the first transmissionbeam as the preferred transmission beam and a first reception beam as apreferred reception beam using the plurality of reference signalstransmitted by the transmitting node.
 7. The method of claim 6, whereinthe determining of whether the change of the transmission beam isnecessary comprises: generating a first beam list representing acorrespondence relationship between transmission beams and correspondingreception beams maximizing the received signal strength if applying eachtransmission beam by using the plurality of reference signals;generating a second beam list using reference signals other than theplurality of reference signals used for generating the first beam list;determining whether a beam pair exists that provides a stronger receivedsignal strength than a received signal strength provided by the firsttransmission beam among one or more beam pairs comprising the firstreception beam in the second beam list; and if a second transmissionbeam included in the beam pair is different from the first transmissionbeam, determining that the first transmission beam is not thetransmission beam maximizing the received signal strength.
 8. The methodof claim 6, wherein the determining of whether the change of thetransmission beam is necessary comprises: determining a secondtransmission beam maximizing the received signal strength with respectto the first reception beam using reference signals other than theplurality of reference signals used for determining the preferredtransmission beam; and if the second transmission is different from thefirst transmission beam, determining that the first transmission beam isnot the transmission beam maximizing the received signal strength. 9.The method of claim 6, wherein the determining of whether the change ofthe transmission beam is necessary comprises determining whetherreceived signal strength of the first transmission beam is less than athreshold.
 10. The method of claim 6, further comprising, if the changeof the transmission beam is determined to be necessary, reselecting thepreferred transmission beam in a range where the preferred receptionbeam is the same.
 11. The method of claim 1, wherein the determining ofthe first transmission beam as the preferred transmission beam using theplurality of reference signals transmitted by the transmitting nodecomprises: measuring a signal strength of each of the plurality ofreference signals transmitted by the transmitting node; and selectingthe first transmission beam, which is the transmission beam with astrongest signal strength from the measured signal strengths, as thepreferred transmission beam.
 12. The method of claim 11, wherein thepreferred transmission beam information comprises an index of thepreferred transmission beam.
 13. The method of claim 11, furthercomprising, if the change of the transmission beam is determined to benecessary, determining a second transmission beam as the preferredtransmission beam, wherein the beam change request includes an index ofthe preferred transmission beam.
 14. The method of claim 1, wherein thedetermining of the first transmission beam as the preferred transmissionbeam using the plurality of reference signals transmitted by thetransmitting node comprises: measuring a signal strength of each theplurality of reference signals transmitted by the transmitting node; andselecting N transmission beams, which correspond to transmission beamswith strongest signal strengths from the measured signal strengths, asthe preferred transmission beams, wherein the first transmission beam isthe transmission beam with the strongest signal strength among the Ntransmission beams.
 15. The method of claim 14, wherein the preferredtransmission beam information comprises an index of the preferredtransmission beams and measurements corresponding to the preferredtransmission beams.
 16. The method of claim 14, wherein the beam changerequest only includes an indication that the beam change is necessary.17. A method for operating a transmitting node in a wirelesscommunication system, the method comprising: transmitting a plurality ofreference signals through corresponding transmission beams; receivingpreferred transmission beam information from a receiving node, which wasgenerated by the receiving node based on the reference signals;identifying a first transmission beam as a transmission beam fortransmitting a signal to the receiving node based on the preferredtransmission beam information; transmitting the signal to the receivingnode via the first transmission beam; receiving a beam change requestfrom the receiving node; identifying a second transmission beam as thetransmission beam for transmitting the signal to the receiving node; andone of adding and changing the second transmission beam as thetransmission beam used for transmitting the signal to the receivingnode.
 18. The method of claim 17, wherein the one of adding and changingof the second transmission beam as the transmission beam used fortransmitting the signal to the receiving node comprises: determiningwhether beam addition is possible; if the beam addition is determined tobe possible, adding the second transmission beam as the transmissionbeam used for transmitting the signal to the receiving node; and if thebeam addition is determined not to be possible, changing thetransmission beam used for transmitting the signal to the receiving nodefrom the first transmission beam to the second transmission beam. 19.The method of claim 18, wherein the determining of whether the beamaddition is possible comprises determining whether at least one RadioFrequency (RF) chain is additionally allocatable to the signaltransmitted to the receiving node.
 20. The method of claim 18, whereinthe determining of whether the beam addition is possible comprisesdetermining whether the beam addition is possible based on the preferredtransmission beam information received from the receiving node.
 21. Themethod of claim 17, wherein the preferred transmission beam informationincludes an index of a preferred transmission beam, and wherein thefirst transmission beam is identified based on the index of thepreferred transmission beam included in the preferred transmission beaminformation.
 22. The method of claim 21, wherein the beam change requestincludes an index of the preferred transmission beam, and wherein thesecond transmission beam is identified based on the index of thepreferred transmission beam included in the beam change request.
 23. Themethod of claim 17, wherein the preferred transmission beam informationincludes an index of a plurality of preferred transmission beams andmeasurements corresponding to the plurality of preferred transmissionbeams, and wherein the first transmission beam is identified based onthe index of the plurality of preferred transmission beams andmeasurements corresponding to the plurality of preferred transmissionbeams.
 24. The method of claim 23, wherein the beam change request onlyincludes an indication that the beam change is necessary, and whereinthe second transmission beam is identified based on the index of theplurality of preferred transmission beams and measurements correspondingto the plurality of preferred transmission beams.
 25. A receiving nodein a wireless communication system, the receiving node comprising: acontroller for controlling a transmitter and a receiver, for determininga first transmission beam as a preferred transmission beam using aplurality of reference signals transmitted by a transmitting node, forgenerating preferred transmission beam information, for controlling totransmit the preferred transmission beam information to the transmittingnode, for controlling to receive transmissions from the transmittingnode via the first transmission beam, for determining whether a changeof a transmission beam is necessary, and if the change of thetransmission beam is determined to be necessary, for generating a beamchange request and for controlling to transmit the beam change requestto the transmitting node; the transmitter for transmitting the preferredtransmission beam information and the beam change request to thetransmitting node; and the receiver for receiving the transmissions fromthe transmitting node.
 26. The receiving node of claim 25, wherein thecontroller tries to decode data to which a Hybrid Automatic RepeatreQuest (HARQ) technique has been applied, and if the decoding fails andthe first transmission beam is not a transmission beam maximizingreceived signal strength, determines that the change of the transmissionbeam is necessary.
 27. The receiving node of claim 25, wherein if thefirst transmission beam is not a transmission beam maximizing receivedsignal strength, the controller determines that the change of thetransmission beam is necessary.
 28. The receiving node of claim 25,wherein a condition under which the change of the transmission beam isnecessary comprises at least one of a mismatch between the firsttransmission beam and the transmission beam maximizing received signalstrength, a failure in decoding data, and a condition in which receivedsignal strength is less than a threshold if applying the firsttransmission beam.
 29. The receiving node of claim 25, wherein todetermine whether change of the transmission beam is necessary, thecontroller determines whether the first transmission beam is atransmission beam maximizing received signal strength.
 30. The receivingnode of claim 25, wherein, if determining the first transmission beam asthe preferred transmission beam using the plurality of reference signalstransmitted by the transmitting node, the controller determines thefirst transmission beam as the preferred transmission beam and a firstreception beam as a preferred reception beam using the plurality ofreference signals transmitted by the transmitting node.
 31. Thereceiving node of claim 30, wherein the controller generates a firstbeam list representing a correspondence relationship betweentransmission beams and corresponding reception beams maximizing thereceived signal strength if applying each transmission beam by using theplurality of reference signals, generates a second beam list usingreference signals other than the plurality of reference signals used forgenerating the first beam list, determines whether a beam pair existsthat provides a stronger received signal strength than a received signalstrength provided by the first transmission beam among one or more beampairs comprising the first reception beam in the second beam list, andif a second transmission beam included in the beam pair is differentfrom the first transmission beam, determines that the first transmissionbeam is not the transmission beam maximizing the received signalstrength.
 32. The receiving node of claim 30, wherein the controllerdetermines a second transmission beam maximizing the received signalstrength with respect to the first reception beam using referencesignals other than the plurality of reference signals used fordetermining the preferred transmission beam, and if the secondtransmission beam is different from the first transmission beam,determines that the first transmission beam is not the transmission beammaximizing the received signal strength.
 33. The receiving node of claim30, wherein to determine whether the change of the transmission beam isnecessary, the controller determines whether received signal strength ofthe first transmission beam is less than a threshold.
 34. The receivingnode of claim 30, wherein, if the change of the transmission beam isdetermined to be necessary, the controller reselects the preferredtransmission beam in a range where the preferred reception beam is thesame.
 35. The receiving node of claim 25, wherein if determining thefirst transmission beam as the preferred transmission beam using theplurality of reference signals transmitted by the transmitting node, thecontroller measures a signal strength of each of the plurality ofreference signals transmitted by the transmitting nodes, and selects thefirst transmission beam, which is the transmission beam with a strongestsignal strength from the measured signal strengths, as the preferredtransmission beam.
 36. The receiving node of claim 35, wherein thepreferred transmission beam information comprises an index of thepreferred transmission beam.
 37. The receiving node of claim 35, furtherwherein, if the change of the transmission beam is determined to benecessary, the controller determines a second transmission beam as thepreferred transmission beam, and wherein the beam change requestincludes an index of the preferred transmission beam.
 38. The receivingnode of claim 25, wherein, if determining the first transmission beam asthe preferred transmission beam using the plurality of reference signalstransmitted by the transmitting node, the controller measures a signalstrength of each the plurality of reference signals transmitted by thetransmitting node, and selects N transmission beams, which correspond totransmission beams with strongest signal strengths from the measuredsignal strengths, as the preferred transmission beams, and wherein thefirst transmission beam is the transmission beam with the strongestsignal strength among the N transmission beams.
 39. The receiving nodeof claim 38, wherein the preferred transmission beam informationcomprises an index of the preferred transmission beams and measurementscorresponding to the preferred transmission beams.
 40. The receivingnode of claim 38, wherein the beam change request only includes anindication that the beam change is necessary.
 41. A transmitting node ina wireless communication system, the transmitting node comprising: abeam-forming unit for applying one or more transmission beams for thetransmission of a plurality of reference signals to a receiving node andthe transmission of a signal to the receiving node; a receiver forreceiving preferred transmission beam information and a beam changerequest from the receiving node; and a controller for controlling thebeam-forming unit and the receiver, for controlling to transmit theplurality of reference signals through corresponding transmission beams,for controlling to receive the preferred transmission beam informationfrom the receiving node, which was generated by the receiving node basedon the reference signals, for identifying a first transmission beam as atransmission beam for transmitting the signal to the receiving nodebased on the preferred transmission beam information, for controlling totransmit the signal to the receiving node via the first transmissionbeam, for controlling to receive the beam change request from thereceiving node, for identifying a second transmission beam as thetransmission beam for transmitting the signal to the receiving node, andfor one of adding and changing the second transmission beam as thetransmission beam used for transmitting the signal to the receivingnode.
 42. The transmitting node of claim 41, wherein the controllerdetermines whether beam addition is possible, if the beam addition isdetermined to be possible, adds the second transmission beam as atransmission beam used for transmitting the signal to the receivingnode, and if the beam addition is determined not to be possible, changesthe transmission beam used for transmitting the signal to the receivingnode from the first transmission beam to the second transmission beam.43. The transmitting node of claim 42, wherein to determine whether thebeam addition is possible, the controller determines whether at leastone Radio Frequency (RF) chain is additionally allocatable to the signaltransmitted to the receiving node.
 44. The transmitting node of claim42, wherein the controller determines whether the beam addition ispossible based on the preferred transmission beam information receivedfrom the receiving node.
 45. The transmitting node of claim 41, whereinthe preferred transmission beam information includes an index of apreferred transmission beam, and wherein the controller identifies thefirst transmission beam based on the index of the preferred transmissionbeam included in the preferred transmission beam information.
 46. Thetransmitting node of claim 45, wherein the beam change request includesan index of the preferred transmission beam, and wherein the controlleridentifies the second transmission beam based on the index of thepreferred transmission beam included in the beam change request.
 47. Thetransmitting node of claim 41, wherein the preferred transmission beaminformation includes an index of a plurality of preferred transmissionbeams and measurements corresponding to the plurality of preferredtransmission beams, and wherein the controller identifies firsttransmission beam based on the index of the plurality of preferredtransmission beams and measurements corresponding to the plurality ofpreferred transmission beams.
 48. The transmitting node of claim 47,wherein the beam change request only includes an indication that thebeam change is necessary, and wherein the controller identifies thesecond transmission beam based on the index of the plurality ofpreferred transmission beams and measurements corresponding to theplurality of preferred transmission beams.